Chinese Patent Application Publication No. CN1045851A, published on Oct. 3, 1990 and entitled "face seal with spiral grooves having "" pattern", and U.S. Pat. No. 5,201,531, published on Apr. 13, 1993 and entitled "face seal with double spiral grooves", each discloses a pure hydrodynamic face seal with double spiral grooves. In the face seal, the radial length of a spiral groove located on the high pressure side, i.e. at the upstream, is less than that located on the low pressure side, i.e. at the downstream. In a given rotational direction, the upstream spiral groove set pumps sealing fluid towards the downstream, while the downstream spiral groove set pumps sealing fluid towards the upstream. The radial length of the downstream spiral groove set has to be great enough, so that the pumping pressure produced by the downstream set can balance the sum of the pumping pressure produced by the upstream set and the pressure difference between the fluid inside and outside the interface. That is to say, the pumping pressure difference produced by these two spiral groove sets must be balanced with the pressure difference between the fluid inside and outside the interface, in order that the leakage of the sealing fluid is ensured to be zero theoretically. However, the kind of seal is only suitable for the condition where the fluid pressure difference across the interface is rather low and viscosity of the sealing fluid is rather high.
All conventional combined hydrodynamic and hydrostatic face seals with spiral grooves are those with monoline spiral grooves. There is an annular flat belt at the downstream of the spiral grooves, which provides resistance to flow, i.e. "obstruction" role. A similar face seal is disclosed in U.S. Pat. No. 4,212,475.
The object of the present invention is to provide a combined hydrodynamic and hydrostatic face seal with double spiral grooves, which forms a fluid film with greater thickness than the prior art under the same condition, or with greater stiffness under the same thickness of the fluid film.